The ever-increasing capacity of small form factor memory cards allows for new possibilities in storing and distributing digital content. Content stored on commercially available cards such as MultiMedia cards (MMC) and Secure Digital (SD) cards may be accessed by a variety of host devices. The organizations that define standards for small form factor memory cards may set limits for the maximum instantaneous or average power that a memory card may consume. These limits are necessary so that manufacturers of host devices such as cellular telephones may budget the instantaneous or average power necessary for memory card access operations, and are necessary in order to maintain interoperability with future and legacy host devices.
As the storage capacity and complexity of small form factor memory cards increase, the instantaneous or average power potentially consumed by these devices also could increase, particularly during power-intensive operations such as reset, programming, writing, or erasing. For example, small form factor memory cards typically contain multiple NAND non-volatile memory dies. The flash memory ready/busy signal, FRDY, is a wired-OR combination of individual open drain outputs driven by non-volatile memory dies. The FRDY signal is driven to a logic low state by one or more dies to indicate that at least one non-volatile memory die is busy and not ready to process another command. A die that is not busy configures its FRDY output to a high-impedance state, thereby allowing other circuit elements to determine the FRDY wired-OR circuit output. When all of the NAND non-volatile memory dies are no longer busy, FRDY is pulled to a logic high state by a pull-up resistor to indicate that the dies are ready to process another command. Driving FRDY to a logic low state to indicate a busy status causes current to sink across the FRDY pull-up resistor. The amount of current sunk is dependent on the resistor value and the voltage drop across the resistor.
This current consumption across the pull-up resistor occurs at an inopportune time, because non-volatile memory dies may be performing a current-intensive operation such as a reset, erase, or programming at the same time that current is being sunk through the FRDY resistor in order to indicate that the dies are busy and not ready to process another command. The current sunk through the FRDY resistor may cause a memory card architecture to exceed an instantaneous or average current consumption established by a small form factor memory card standard. Existing non-volatile memory systems typically do not account for the additional current sunk through the FRDY pull-up resistor during NAND non-volatile memory die operation and have not addressed how to effectively reduce it without adversely impacting system operation.
One design approach to reduce the current consumption through a FRDY pull-up resistor is to increase its resistance. However, increasing the resistance of the pull-up resistor may slow the FRDY rising edge transition from a logic low state to a logic high state, which may adversely impact system operation and/or system performance. Incorporating a physically larger discrete resistor into the non-volatile storage device may increase the cost of the device. If the resistor is integrated into the non-volatile memory controller, increasing the resistance of the pull-up resistor may also increase the die area required for the controller.
Another design approach to reducing current consumption involves eliminating or disabling the inputs to the FRDY wired-OR circuit and polling the busy/ready status from each non-volatile memory die. However, polling each non-volatile memory die may increase the system power consumption. Moreover, disabling the open drain transistors in the FRDY wired-OR circuit may necessitate customized logic in the non-volatile memory die that is not available from every die manufacturer.